Electrophotographic dry developers are divided into one-component developers comprising a toner itself containing a binder resin having dispersed therein a colorant and two-component developers comprising a toner and a carrier. In carrying out electrophotography using either type of the developer, an electrostatic latent image formed on a photoreceptor is visualized with the developer to form a toner image, which is then transferred to a transfer material, such as paper or a sheet, and fixed by means of heat, a solvent, pressure, etc. Thereafter, the photoreceptor is cleaned to remove any remaining toner.
Accordingly, a dry developer is required to satisfy various conditions in electrophotography, particularly in the development step or cleaning step. That is, a toner should be used not in the form of agglomerates but in the form of independent particles. To this effect, it is required that the toner should have sufficient fluidity and that the flow characteristics or electrical characteristics of the toner should not be changed with time or change in environmental conditions such as temperature and humidity. Further, the toner on the photoreceptor should be completely transferred to a transfer material, or if it remains thereon, the residual toner should be completely removed from the photoreceptor by a cleaning step.
In addition, the toner in a two-component developer is required to cause no filming phenomenon, i.e., caking of a toner, on the surface of carrier particles.
Development and transfer each consist in, in principle, relieving toner particles of the bonds with a substrate supporting them and adhering them to another substrate (i.e., a photoreceptor or a transfer material), while somewhat influenced by uniformity in the developer flow or the electric current at the time of transfer. Therefore, these steps greatly depend on the balance between static attraction and adhesion between toner particles and a charge-imparting member or adhesion between toner particles and a photoreceptor. While this balance is very difficult to control, there is a demand for high performance in these steps because these steps have direct influences on image quality and also because an improvement in efficiency of these steps is expected to bring about improved reliability and energy saving in cleaning.
The development or transfer takes place when static attraction force is greater than adhesive force. Therefore, improvement in efficiency in these steps may be achieved either by increasing the static attraction force (i.e., by enhancing a developing or transferring force) or by decreasing the adhesive force. An increase in developing or transferring force through, for example, an increase in electrical field for transfer, is apt to cause secondary troubles, such as occurrence of toner particles bearing opposite polarity. Accordingly, reduction of adhesive force is a more effective approach.
The adhesive force includes Van der Waals force (non-static adhesion force) and mirror image force of charges possessed by toner particles. The former being far greater than the latter nearly by one figure, it can consider the adhesive force in terms of Van der Waals force.
Van der Waals force F among spherical particles is represented by equation: EQU F=H.multidot.r.sub.1 .multidot.r.sub.2 /6(r.sub.1 +r.sub.2).multidot.a.sup.2
wherein H represents a constant; r.sub.1 and r.sub.2 each represent a radius of particles in contact with each other; and a represents a distance between the particles.
In order to reduce the adhesive force, it has been proposed to incorporate fine particles having a much smaller radius than toner particles between toner particles and the surface of a photoreceptor or a charge-imparting member thereby to separate them apart with a distance and to reduce the contact area (the number of contact points). In this connection, addition of various additives, such as inorganic compounds, e.g., silica, alumina and zinc oxide, to a toner has been suggested for improvements in development and transfer efficiency, fluidity, durability, or cleanability.
With respect to cleaning, the toner remaining on a photoreceptor should easily be removed therefrom. Where a cleaning element, such as a blade or a web, is used in the cleaning step, scratching of the photoreceptor with such an element should be avoided.
For the purpose of meeting these requirements, it has been proposed to add to a one-component or two-component developer various external additives for improving fluidity, durability or cleanability, such as inorganic powders (e.g., silica) and organic powders (e.g., fatty acids, fatty acid metal salts, and derivatives thereof, styrene-acrylic resins, olefin resins, and fluorine-containing resins).
Of the additives proposed to date, inorganic powders, such as silica, alumina, and zinc oxide, considerably improve fluidity of dry developers, as described in JP-A-59-226355, JP-A-61-23160, JP-A-63-118757, JP-A-2-1870, and JP-A-2-90175 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, because of their hardness and irregularity in shape, they are liable to separate from toner particles and cause scratches on the surface of a photoreceptor upon cleaning. It easily follows that toner particles cake onto the scratched part of the photoreceptor. Further, these fine powders tend to migrate to the surface of the photoreceptor and adhere thereto to form nuclei on which resins, etc. are accumulated in the meantime to cause troubles, such as formation of black spots on the photoreceptor.
Recently, size reduction of toner particles has been promoted for the purpose of improving image quality, reducing a pile-height of toner image in a digital imaging device, or reducing toner consumption. Size reduction of toner particles, however, leads to an increase in average number of contact points per unit weight, and in accordance therewith the Van der Waals force increases, which entertains a fear of considerable reductions in toner fluidity and development and transfer performance.
In order to overcome this problem, it has been studied to increase the amount of the inorganic compound to be added or to use a transfer-improving agent in combination, as disclosed in JP-A-63-279263 and JP-A-63-279264. However, such a solution arouses secondary problems due to release of the additive, such as charging disorder and filming on the photoreceptor.
It has also been proposed to apply a mechanical impact so as to firmly adhere an externally added additive, such as an inorganic compound, to the surface of toner particles and to prevent the additive from migrating to a photoreceptor, a cleaning element, etc., as disclosed, e.g., in JP-A-2-167561. This technique is effective to alleviate the troubles arising from release of the inorganic compound (e.g., formation of black spots on the photoreceptor) but, on the other hand, results in serious impairment of fluidity.
In recent years, a magnetic one-component development system has been regarded with expectation of high reliability, which will exclude the necessity of maintenance, and size reduction or simplification of an electrophotographic device. However, a magnetic one-component development system is liable to cause troubles characteristic of a magnetic powder, such as wear or damage of a photoreceptor with a released magnetic powder released from toner particles or exposed on the surface of the toner particles.
Recycled paper has been steadily extending its use with the aim of resources-saving. In general, recycled paper generates much paper dust, and the paper dust tends to enter the gap between a photoreceptor and a cleaning blade, causing cleaning defects, such as black streaks.
In order to overcome these problems, external addition of fatty acid metal salts (as described in JP-A-59-187347 and JP-A-60-198556) or waxes, e.g., polyethylene wax, (as described in JP-A-55-12977, JP-A-61-231562, and JP-A-61-231563) as a lubricant has been studied.
Any of these external additives proposed has a large particle size of from 3 to 20 .mu.m. Accordingly, they should be added in a considerable amount to be made efficient use of. Besides, although these lubricants are effective in the initial stage, they themselves undergo a filming phenomenon, failing to form a uniform lubricating film, causing image defects, such as white spots and blurs.
A cleaning system using a rubber blade, a brush, etc. has been employed particularly where an organic photoreceptor in a belt form (hereinafter referred to as a "belt photoreceptor") is used in a high-speed copying machine. As compared with a drum photoreceptor, cleanability of an organic belt photoreceptor is largely affected by its distortion or sag. Therefore, cleaning of an organic belt photoreceptor must be carried out under a high load of a blade upon the photoreceptor. Further, the state-of-the-art belt photoreceptors have seams, at which a blade chatters or a blade is scratched to cause poor cleaning. Addition of the above-mentioned additives to the toner has been examined for applicability to such a belt photoreceptor system. It was revealed as a result that the particles added are easily deformed under strong shearing in every case. That is, the additives are effective in the initial stage but undergo a filming phenomenon itself before long to cause white spots, blurs, etc.
In an attempt to improve fixing so as to cope with an increased copying speed, use of a styrene-acrylic copolymer having two molecular weight peaks as a binder resin of a toner has been proposed, as disclosed in JP-A-1-204061. However, due to the presence of a low-molecular weight polymer component, the toner has low rheological characteristics and undergoes deformation due to shearing on cleaning. As a result, the toner tends to undergo a filming phenomenon on the photoreceptor or adhesion or fusion with a carrier (hereinafter referred to as "impaction"), leading to a reduction of working life of the developer.
It has been suggested to add hydrophobic hard fine particles to a toner so that a photoreceptor is abraded by the hard fine particles to prevent toner filming as disclosed in JP-A-2-89064. While effective to prevent a filming phenomenon, the hard particles added wear the surface of a photoreceptor, resulting in a serious reduction in durability of the photoreceptor. A cleaning blade is also worn out by the hard fine particles.
In addition, since the surface of inorganic oxide particles is generally covered with a hydroxyl group, they adsorb moisture under a high humidity condition when added as an external additive, exerting influences on the charge quantity or fluidity of the toner. That is, addition of inorganic oxide particles results in increase in dependence on environmental conditions.
With respect to magnetic powders, surface treatment with a titanium coupling agent, etc. for improving compatibility with a binder resin (as described in JP-A-56-51755, JP-A-60-121457, and JP-A-60-166959), application of mechanical force for adhering the magnetic powder to the surface of toner particles or for burying the magnetic powder in the toner particles thereby reducing exposed or free magnetic powders (as described in JP-A-64-93748), or use of a spherical magnetic powder for reducing the stress with a photoreceptor have been suggested.
In order to meet the recent demands for low potential, high development performance, and high image quality, several techniques for increasing mobility of a developer have been proposed. Among them is the use of a developer composed of a toner and a low-specific gravity, dispersion type carrier essentially comprising a resin and a magnetic powder. However, the low-specific gravity and low-magnetic force dispersion type carrier is apt to adhere to the surface of a photoreceptor and give scratches thereto on cleaning.